Watercolor paintings should be matted and framed under glass. It is important to use a mat when framing your watercolors. The mat keeps the painting away from the glass and creates a bit of air space between the glass and the mat. I always use acid free mats on my paintings. The watercolors are mounted to either the mat or the backer with tape hinges. This allows for natural expansion and contraction of the paper through out the year with the change in humidity. If possible a brown paper is taped to the back of the frame to keep dust out.